The present invention relates generally to dry etching, and more particularly to a method for restoring wafer properties after dry etch processing.
Dry etching, and in particular, reactive ion etching, has become a key process for the fabrication of microscale devices in the semiconductor industry. Unfortunately, it has been established that dry etch exposure modifies the surface properties of the semiconductor being etched due to energetic ion bombardment damage and residue contamination.
Studies have shown that there can be a residue layer of foreign material formed on the surface of a reactively ion etched semiconductor wafer resulting from the chemical reactions involved in some RIE processes.
Recent studies have also shown that underlying the surface of a reactively ion etched semiconductor wafer there is a damaged layer in the near surface region which contains crystalline defects, such as dislocations, vacancies, and interstitials and which may contain impurities such as etching species. The depth of this layer may vary from tens of angstroms to several hundred angstroms, depending on the ion energies used. If this damaged region is not restored or removed completely by post RIE cleaning steps, it can have deleterious effects on device characteristics. Standard post cleaning methods, such as O.sub.2 plasma ashing, are not effective in eliminating such crystalline damage. It is possible to remove this damaged region by means of a sacrificial oxide film growth or by a wet chemical etching of the semiconductor material. However, the complete elimination of this damaged region by these techniques can be unacceptable because the material removal is incompatible with the micron/submicron dimensions of current devices. An alternate method of correcting RIE crystalline damage via high temperature furnace annealing of the semiconductor wafers is not acceptable due to its tendency to cause significant atomic diffusion. Such high temperature diffusion destroys the geometries required for current semiconductor devices.
A number of different methods have been suggested in the art for avoiding these drawbacks while recovering the surface properties of a semiconductor surface exposed to RIE. For example, a short-time annealing process was suggested in Ransom et al, the Fifth International Symposium on Plasma Processing, Oct. 7-14, 1984. Ransom et al utilized short-time annealing at 950.degree., 1050.degree., and 1150.degree. C., after CClF.sub.3 /H.sub.2 RIE and after CClF.sub.3 /H.sub.2 RIE with an O.sub.2 -RIE/HF dip surface preparation on a silicon wafer. Ransom et al determined that this type of processing was not effective in restoring diode properties. In contrast, Fonash et al, Journal of Applied Physics 58(2), 15 July 1985, at p. 862, successfully utilized a rapid thermal anneal in order to partially restore silicon surface properties damaged by a CCl.sub.4 -RIE. In view of the unsuccessful restoration of crystallinity in Ransom et al, and the partially successful crystallinity restoration in Fonash et al, it was speculated that the rapid thermal anneal which was performed in both studies must be done on semiconductor surfaces which have no substantial residue or film layer thereon. In this regard, it is well known that chloro-fluoro-carbon gas, of the type used in Ransom et al, forms a contaminated film on the surface of silicon, which film contains carbon, chlorine, and fluorine atoms. Accordingly, for chloro-fluoro-carbon reactive ion etched wafers, the contaminating carbonaceous layer formed thereon after the RIE was removed by the present inventors prior to performing a rapid thermal annealing step. However, when this procedure for restoring the surface was used, an unexpectedly large series resistance was obtained in diodes formed on this etched semiconductor material, thereby indicating that the semiconductor surface was not fully recovered.
The invention as claimed is intended to remedy the above-described contamination removal and crystallinity restoration problems. The invention solves the problem of restoring semiconductor surface properties after CClF.sub.3 /H.sub.2 RIE, and enhances surface property and crystallinity restoration for other RIE gas mixtures.